Admin Menu - Highlight top-level menu when on a sub-menu page (without showing sub-menu)

Question

This is the current situation:

• I am writing a plugin that has a top-level menu page
• The plugin also contains other sub-menu pages
• The sub-menu pages are not assigned a parent slug (thus they do not appear in the admin menu)

My requirement is to do the following:

• Have only the top-level menu page menu item in the admin menu (no sub-menu page menu items should appear)
• Highlight the top-level menu page menu item when on a sub-menu page

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I've tried giving a parent slug to the sub-menu pages. This, of course, makes it appear as expected under the top-level menu item. I then tried to remove the sub-menu item, by using remove_submenu_page. However, since that removes the entire sub-menu page, it doesn't solve the problem.

I believe the logical thing to do, is to give a parent slug, since the top-level menu item will need to know that the page being accessed falls under it. The problem then, is not showing the sub-menu page's menu item. I am unable to find a function that deals directly with the menu items.

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Is this the right approach or is there a better alternative? Is there a action/filter/hook I can utilise to carry out what I need to do?

Edit #1:
Did some further research/testing. Setting the menu_title parameter to null or '' in the add_submenu_page call makes the title not appear. However, the <li> that contains the sub-menu item still does exist and can be interacted with. When there are numerous sub-menu pages (as in my case), the empty <li> elements will add-up and create a long blank area on the menu. This is far from ideal and as such, I'm still searching for a solution.

Edit #2: Tried manipulating the $submenu global after the comment from @s_ha_dum. However, any removal of the menu section pertaining to a page also results in removing the page similar to remove_submenu_page and thus rendering that page inaccessible.

Answer 1

That's a bit of a late answer and I don't know if @Jay ever sorted it out, but to anyone having the same issue, here's how I fixed it.

Menu Pages

function my_admin_menu() {

    add_menu_page(
        'Page title',
        'Menu title',
        'manage_options',
        'my_page',
        null,
        null,
        99
    ); 

    add_submenu_page(
        'my_page',
        'Subpage 1 title',
        'Subpage 1 menu title',
        'manage_options',
        'my_subpage_1',
        null
    );

    add_submenu_page(
        'my_page',
        'Subpage 2 title',
        'Subpage 2 menu title',
        'manage_options',
        'my_subpage_2',
        null
    );
}
add_action( 'admin_menu', 'my_admin_menu' ) );

function my_admin_head() {

    remove_submenu_page( 'my_page', 'my_subpage_1' );
}
add_action( 'admin_head', 'my_admin_head' );

See the Codex for remove_submenu_page.